Multi-element planar array antenna

ABSTRACT

A multi-element planar array antenna has a substrate made of a dielectric material or the like; a planar conductor formed on a first principal surface of the substrate; a first and a second slot line formed in the conductor and intersecting each other; a first and a second microstrip line formed on a second principal surface of the substrate, and traversing the first slot line respectively at positions corresponding to both end sides of the first slot line; a third and a fourth microstrip line formed on the second principal surface, and traversing the second slot line respectively at positions corresponding to both end sides of the second slot line; and four slot line antenna elements formed on the first principal surface respectively in intersection regions between both end sides of the first and second microstrip lines and both end sides of the third and fourth microstrip lines.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a multi-element planar arrayantenna which comprises a plurality of antenna elements arranged on atwo-dimensional plane, and more particularly, to a multi-element planararray antenna which facilitates the utilization of polarizationcomponents, and can be readily reconfigured into an active antenna bymounting thereon a semiconductor device, IC (integrated circuit) and thelike.

[0003] 2. Description of the Related Arts:

[0004] Planar antennas are widely used in, for example, radiocommunications, satellite broadcasting in a microwave band and amillimeter band. Planar antennas are classified into a microstrip linetype, a slot line type, and the like. Generally, the microstrip lineplanar antenna is often used because of a simple structure in a feedsystem and the like. The slot line planar antenna is advantages in thatit operates in a wide frequency band, readily suppresses an orthogonalcomponent, and the like. In recent years, a so-called multi-elementarray structure using a plurality of antenna elements has been employedwith the intention of improving the antenna gain which is a challengefor the microstrip line planar antenna.

[0005] As is well known, electromagnetic radiations include polarizationcomponents such as horizontal and vertical linear polarizations, andright-handed and left-handed circular polarizations. Many antennasmaking use of such polarization characteristics are widely used with theintention of, for example, sharing an antenna for transmission andreception, effectively utilizing the frequency resources, suppressinginterference between transmission and reception.

[0006]FIGS. 1A to 1D are plan views respectively illustrating exemplaryconfigurations of conventional planar antennas. Out of the illustratedplanar antennas, those illustrated in FIGS. 1A, 1B and 1C are microstripline planar antennas, while that illustrated in FIG. 1C is a slot lineplanar antenna. Each of these figures illustrates an exemplaryconfiguration of a planar antenna having a single antenna element forproducing a linear or a circular polarization.

[0007] The planar antenna illustrated in FIG. 1A is a microstrip lineplanar antenna for linear polarization which comprises square antennaelement (i.e., circuit conductor) 21 and feed line 22 on one principalsurface of substrate 23 made, for example, of a dielectric material. Aground plane conductor is disposed substantially over the entirety ofthe other principal surface of substrate 23. In this planar antenna, theantenna frequency (resonant frequency) is determined by the shape ofantenna element 21, the dielectric coefficient of substrate 23, and thelike. Also, in this planar antenna, a polarization plane of linearpolarization for transmission and reception is set by a feedingdirection in which feed line 22 is connected with respect to antennaelement 21. Specifically, as indicated by arrows, a verticalpolarization component can be transmitted and received when antennaelement 21 is fed in the vertical direction (up-to-down direction in thefigure), while a horizontal polarization component can be transmittedand received when antenna element 21 is fed in the horizontal direction(left-to-right direction in the figure).

[0008] The planar antenna illustrated in FIG. 1B is microstrip lineplanar antenna having square antenna element 21 on one principal surfaceof substrate 23, similar to the one illustrated in FIG. 1A, but differsin that antenna element 21 is fed at two points so that it is adaptedfor use with a circular polarization. Specifically, feed line 22 isbranched into two in the middle such that one of the branch lines isused as a feed line for a vertical polarization component while theother is used as a feed line for a horizontal component. The feed linesfor respective components differ in the electric length from each otherby one quarter wavelength. As a result, a vertical polarizationcomponent is out of phase from a horizontal polarization component by 90degrees (π/2), so that these polarization components are combined into acircular polarization. Consequently, the resulting planar antenna iscapable of transmitting and receiving a circular polarization. It shouldbe noted that the planar antennas illustrated in FIGS. 1A and 1B eachutilize a degeneration mode in antenna element 21.

[0009] The planar antenna illustrated in FIG. 1C is a microstrip lineplanar antenna for circular polarization, in which degeneration isreleased in antenna element 21 to feed antenna element 21 at one point.In this planar antenna, portions of square antenna element 21 in a setof diagonal directions are cut away to release the degeneration so thatresonance modes in two directions (vertical and horizontal directions)are out of phase by 90 degrees from each other at the operatingfrequency of the antenna, thereby providing the capabilities to transmitand receive a circular polarization.

[0010]FIG. 1D illustrates a slot line planar antenna for use with acircular polarization which releases degeneration in an antenna element.This planar antenna comprises antenna element 24 formed as a slot linethat circumvents on one principal surface of substrate 23, instead of anantenna element in a microstrip line planar antenna. Antenna element 24is rectangular in shape and released from the degeneration, therebyconstituting a resonator at the antenna frequency. When antenna element24 is fed at one corner thereof, resonance modes in the two directionsare out of phase by 90 degrees from each other, similar to theforegoing, thereby providing the capabilities to transmit and receive acircular polarization.

[0011] Each of the conventional microstrip line and slot line planarantennas described above can be shared for a horizontal polarization anda vertical polarization, and transmit and receive the circularpolarization when it is provided with a single antenna element alone.However, these conventional planar antennas are problematic inconfiguring a multi-element planar array antenna comprised of aplurality of antenna elements arranged in a two-dimensional plane whilemaintaining the above functions of the planar antenna having a singleantenna element.

[0012] Specifically, any of the planar antennas of the types illustratedin FIGS. 1A to 1D encounters difficulties, when it is configured as amulti-element array, in implementing connections of the feed line torespective antenna elements, i.e., a feeder circuit on a single plane.For this reason, a multi-layer substrate, for example, should be used toimplement a feeder circuit, in which case difficult designing is obligedfor ensuring the same line lengths, for example, from a feed point, dueto a requirement of exciting the respective antenna elements in phase.

[0013] Further, when the configuration illustrated in FIG. 1B is usedfor a circular polarization antenna, a phase difference feeder circuitis required for each antenna element to give a phase difference of 90degrees (i.e., π/2). The planar antenna illustrated in FIG. 1C canoperate only in a narrow frequency range on principles.

[0014] Although there is an example of a linear array in which acoplanar line is connected to a feed line, the planar antennaillustrated in FIG. 1D is similar in that it encounters difficulties indouble use of both vertical and horizontal polarization components, andan adaptation for a two-dimensional planar array antenna including acircular polarization.

[0015] As described above, the conventional planar antennas, whicheverone is concerned, generally have a problem in the double use ofpolarizations, and the adaptation for a two-dimensional planar arrayantenna including a circular polarization.

SUMMARY OF THE INVENTION

[0016] It is an object of the present invention to provide amulti-element planar array antenna which has a two-dimensional arraystructure that can use polarizations together and use a circularpolarization.

[0017] The inventors diligently investigated the configuration of planarantennas, and perceived the transmission characteristics and linestructures of microstrip lines and slot lines formed on both sides of asubstrate made of a dielectric material or the like, particularlyperceived features of an anti-phase serial branch from the slot line tothe microstrip line, and a circuit in which slot lines intersect eachother, and reached the completion of the present invention by making themost of these features.

[0018] Specifically, the object of the present invention is achieved bya multi-element planar array antenna which includes a substrate having afirst and a second principal surface, a conductor formed on the firstprincipal surface, a first and a second slot line formed in theconductor, and intersecting each other, a first and a second microstripline formed on the second principal surface, and traversing the firstslot line respectively at positions corresponding to both end sides ofthe first slot line, a third and a fourth microstrip line formed on thesecond principal surface, and traversing the second slot linerespectively at positions corresponding to both end sides of the secondslot line, and four microstrip line antenna elements formed on the firstprincipal surface respectively in intersection regions between both endsides of the first and second microstrip lines and both end sides of thethird and fourth microstrip lines. Each antenna element is arranged forexcitation in two directions by electromagnetically connecting to one ofboth ends of one of the first and second microstrip lines and to one ofboth ends of one of the third and fourth microstrip lines through thesubstrate for excitation in two directions. The two excitationdirections of each antenna element are typically orthogonal to eachother, and each antenna element is excited in phase.

[0019] The substrate for use in the present invention is made, forexample, of a dielectric material. The conductor formed on the firstprincipal surface of the substrate is, for example, a planar metalconductor. This conductor functions as a ground plane for the first tofourth microstrip lines.

[0020] In this multi-element planar array antenna, a feed point istypically at the intersection of the first and second slot lines. Anexcitation mode is selected for each antenna element by selecting atleast two of four corners formed in the conductor at the intersectionand applying a high frequency signal to the selected corners.

[0021] Specifically, in the present invention, the microstrip lines arerouted on both end sides of a set of intersecting slot lines to traversethem, so that a high frequency signal in a balanced mode, propagatingthrough the slot line, is converted to an unbalanced mode by themicrostrip lines, and propagates in anti-phase series branch. Also, anexcitation direction in each antenna element can be selected byselecting corners of the conductor constituting the intersection of theset of the slot lines at the intersection, and applying a high frequencysignal to the selected corners. For example, by selecting corners toapply a high frequency signal between the conductors on both sides ofthe first slot line, the high frequency signal is converted to theunbalanced mode by the first and second microstrip lines, and is fed toeach antenna element in a direction orthogonal to the direction in whichthe first slot line extends. Similarly, by selecting corners to apply ahigh frequency signal between the conductors on both sides of the secondslot line, each antenna element is fed in a direction orthogonal to thedirection in which the second slot line extends. By thus selecting afeed mode at the intersection, one of the first and second slot linescan be excited, and an excitation direction can be selected for eachantenna element. Thus, the multi-element planar array antenna can selectone from linear polarizations in orthogonal directions as well as canuse the linear polarizations together.

[0022] Further, as one pair of corners in a diagonal direction isselected from four corners at the intersection and applied with a highfrequency signal, both slot lines are excited so that each antennaelement is simultaneously fed from the two directions orthogonal to eachother. As such, polarization components in the two directions arecombined to provide a polarization component in an intermediatedirection of the two directions. In addition, when the corners in therespective diagonal directions are formed in pairs, and each pair isapplied with a high frequency signal at a different level, thepolarization direction can be arbitrarily controlled to utilize anypolarization component.

[0023] Moreover, when the first and second slot lines are set such thattheir electric lengths differ from each other by π/2 as calculated interms of phase difference, a circular polarization can be transmittedand received by applying a high frequency signal to one pair of cornersin one diagonal direction at the intersection. For example, a circularpolarization can be generated by delaying a vertical excitationcomponent in phase from a horizontal excitation component by π/2. Inthis event, a radiated electromagnetic wave can be a right-handedcircular polarization or a left-handed circular polarization byselectively applying a high frequency signal to one or the other pair ofcorners positioned in the diagonal directions at the intersection. It istherefore possible to select a circular polarization, and again select aright-handed circular polarization or a left-handed circularpolarization as well as to use the right-handed circular polarizationtogether with left-handed circular polarization by simultaneouslyselecting both circular polarizations, wherein, by way of example, theright-handed circular polarization is transmitted while the left-handedcircular polarization is received, thereby readily implementing amulti-element planar array antenna capable of selecting one fromorthogonal circular polarizations and using them together.

[0024] Moreover, in the present invention, a 16-element planar arrayantenna and planar antenna having a larger number of antenna elementscan be configured by utilizing an in-phase parallel branch of slot linesfrom microstrip lines.

[0025] As appreciated from the foregoing, the present invention canreadily implement a four-element planar array antenna which can usetogether linear polarizations such as a horizontal polarizationcomponent and a vertical polarization component. The present inventioncan also implement a four-element planar array antenna which can usetogether orthogonal circular polarizations such as a right-handedcircular polarization and a left-handed circular polarization. Inaddition, the present invention can readily implement multi-elementplanar array antenna having 8-elements, 16-elements, 64-elements and thelike. The present invention readily implements a planar array antennawhich supports multiple bands by use of two frequencies. In essence, thepresent invention provides a slot line planar array antenna which can bereadily configured as a two-dimensional array that can use multiplepolarizations together or use a circular polarization.

[0026] Since the present invention utilizes the series branches from theslot lines to the microstrip lines, the antenna elements arecomplementary to each other in excitation, consequently providing aplanar antenna which has suppressed orthogonal polarizations and goodcircular polarization axial ratio characteristics. Further, the planarantenna structure of the present invention facilitates mounting of afunctional circuit such as a semiconductor device, an integrated circuit(IC) chip and the like, and therefore is effective in providing anactive planar array antenna, an adaptive active planar array antenna,and a smart planar array antenna.

[0027] Since the multi-element planar array antenna according to presentinvention is based on integration of slot line antenna elements,electromagnetic waves radiate from both principal surfaces of thesubstrate. When a need exists for radiating an electromagnetic wave onlyfrom one of the principal surfaces of the substrate, an electromagneticshielding box, a shielding plate, a reflector or the like may beprovided on the other principal surface of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIGS. 1A to 1D are plan views each illustrating an exemplaryconfiguration of a conventional planar antenna;

[0029]FIG. 2A is a plan view illustrating a slot line multi-elementplanar array antenna according to a first embodiment of the presentinvention;

[0030]FIG. 2B is a cross-sectional view taken along line AA in FIG. 2A;

[0031] FIGS. 3 to 5 are plan views each illustrating an exemplaryoperation of the planar array antenna according to the first embodiment;

[0032]FIG. 6 is a plan view illustrating a slot line multi-elementplanar array antenna according to a second embodiment of the presentinvention;

[0033]FIG. 7 is a plan view illustrating a slot line multi-elementplanar array antenna according to a third embodiment of the presentinvention;

[0034]FIG. 8A is a plan view illustrating a slot line multi-elementplanar array antenna according to a fourth embodiment of the presentinvention;

[0035]FIG. 8B is a cross-sectional view taken along line AA in FIG. 8A;

[0036]FIG. 9A is a plan view illustrating a slot line multi-elementplanar array antenna according to a fifth embodiment of the presentinvention;

[0037]FIG. 9B is a cross-sectional view taken along line A-A in FIG. 9A;

[0038]FIG. 10A is a plan view illustrating a slot line multi-elementplanar array antenna according to a sixth embodiment of the presentinvention;

[0039]FIG. 10B is a cross-sectional view taken along line A-A in FIG.10A;

[0040]FIG. 11A is a plan view illustrating a slot line multi-elementplanar array antenna according to a seventh embodiment of the presentinvention;

[0041]FIG. 11B is a cross-sectional view taken along line A-A in FIG.11A; and

[0042]FIG. 12 is a plan view illustrating a slot line multi-elementplanar array antenna according to an eighth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0043] A slot line multi-element planar array antenna according to afirst embodiment of the present invention, illustrated in FIGS. 2A and2B, comprises planar conductor 2 formed substantially over the entiretyof a first principal surface of substrate 3 made, for example, of adielectric material or the like. Conductor 2 is formed with first andsecond slot line 4 a, 4 b such that they intersect each other at theirrespective midpoints and extend orthogonal to each other. Conductor 2 ismade, for example, of a metal layer or a thin metallic plate. In thefigures, first slot line 4 a extends in the horizontal direction, whilesecond slot line 4 b extends in the vertical direction. Slot lines 4 a,4 b are formed as slot lines having the same length and short-circuitedat both ends, in the shape of a cross as a whole. As will be laterdescribed, the planar antenna is fed at four corners formed at theintersection of slot lines 4 a, 4 b on conductor 2.

[0044] Further, four slot line type antenna elements 1 are disposed onthe first principal surface of substrate 3. Specifically, antennaelements 1 are formed by routing slot lines each of which circumventsalong the outer periphery of a square on conductor 2. Therefore, in asmall square area surrounded by each slot line, conductor 2 stillremains on substrate 3. A conductor in the small square area surroundedby each slot line is called the “central conductor of antenna element1.”

[0045] In FIG. 2A, a dotted portion indicates the location at whichconductor 2 is formed on the first principal surface of substrate 3. InFIG. 2B, fat black line segments represent conductor 2 on the firstprincipal surface and conductors which serve as microstrip lines on thesecond principal surface.

[0046] On the second principal surface of substrate 3, four microstriplines 5 a to 5 d are routed at equal distances from the intersection ofslot lines 4 a, 4 b in the vertical and horizontal directions such thatmicrostrip lines 5 a to 5 d orthogonally traverse slot lines 4 a, 4 b,respectively. The leading end of each slot line 4 a, 4 b, which isshort-circuited to form a termination, is preferably extended byapproximately λ/4 beyond the position across which associated microstripline 5 a to 5 d traverses, where λ is a wavelength corresponding to theantenna frequency of the planar antenna. Therefore, each of the leadingends of each slot line 4 a, 4 b electrically functions as an open end,viewed from the intersection with the associated microstrip line, at theantenna frequency. Conductor 2 formed on the first principal surface ofsubstrate 3 also functions as a ground plane or ground conductor formicrostrip lines 5 a to 5 d.

[0047] All microstrip lines 5 a to 5 d have the same length, and areformed along the sides of a certain square as a whole. Slot line antennaelements 1 are disposed, respectively at positions corresponding to thecorners of the square. Each of microstrip lines 5 a to 5 d formed on thesecond principal surface of substrate 3 has leading end portions each ofwhich overlaps with the central conductor of associated antenna element1 through substrate 3. Specifically, each of leading end of eachmicrostrip line 5 a to 5 d traverses the slot line around the centralconductor of corresponding antenna element 1, and extends below thecentral conductor. In this event, the microstrip line traverses the slotline formed in a square track shape at the center of one side of thesquare. The microstrip line thus traversing the slot line iselectromagnetically coupled to the slot line of antenna element 1,resulting in electromagnetic coupling of the microstrip line withantenna element 1. Consequently, antenna element 1 can be fed from themicrostrip line.

[0048] Antenna element 1 at the upper right corner in the figureoverlaps with the right end of microstrip line 5 c and an upper end ofmicrostrip line 5 b to create electromagnetic coupling with these endsof microstrip lines 5 c, 5 b, so that antenna element 1 is fed at twopoints from these microstrip lines 5 c, 5 b. Similarly, antenna element1 at the lower right corner in the figure is electromagnetically coupledto the right end of microstrip line 5 d and the lower end of microstripline 5 b; antenna element 1 at the upper left corner in the figure iselectromagnetically coupled to the left end of microstrip line 5 c andthe upper end of microstrip line 5 a; and antenna element 1 at the lowerleft corner in the figure is electromagnetically coupled to the left endof microstrip line 5 d and the lower end of microstrip line 5 a.

[0049] In this planar antenna, each antenna element 1 has a degenerationmode in the horizontal and vertical directions orthogonal to each other.The same electronic length is set from the intersection of first andsecond slot lines 4 a, 4 b to respective antenna elements 1 through slotlines 4 a, 4 b and microstrip lines 5 a to 5 d.

[0050] Next, the operation of the planar array antenna will bedescribed. As described above, in this planar antenna, a high frequencysignal is applied at a feed position composed of four corners onconductor 2 which are formed at the position at which first and secondslot lines 4 a, 4 b intersect to each other. For convenience, the fourcorners are designated a, b, c, d in the clockwise direction from theupper left corner in the figure.

[0051] First, among four corners at the feed position, corners a, babove first slot line 4 a are designated as a pair, while corners c, dbelow first slot line 4 a are likewise designated as another pair. Ahigh frequency signal is applied or fed between corners a, b and cornersc, d. In this event, first slot line 4 a is excited by the highfrequency signal applied on both sides, permitting a high frequencycomponent in a balanced mode to propagate through first slot line 4 a.Then, the high frequency component is converted to an unbalanced mode byfirst and second microstrip lines 5 a, 5 b which traverse first slotline 4 a on the left and right end sides, respectively. The convertedhigh frequency component propagates to respective antenna elements 1. Ineach antenna element 1, the high frequency signals from the microstriplines propagate in in-phase parallel branch with respect to the slotlines along which antenna element 1 is formed.

[0052] Since the conversion from the slot line to the microstrip line ismade through an anti-phase series branch, the high frequency componentsconverted to microstrip lines 5 a, 5 b propagate in opposite phase.Since the electric lengths from the intersection of slot lines 4 a, 4 bto respective antenna elements 1 are identical, respective antennas 1are applied with the high frequency signal in opposite phase. However,respective antenna elements 1 are excited in phase because the feedpoints of the antennas are in a mirror symmetry relationship. In thisevent, since respective antennas 1 are fed in the vertical direction, avertical polarization is fed. Also, in this event, since second slotline 4 b is not excited, no high frequency component propagates throughmicrostrip lines 5 c, 5 d.

[0053] Next, as illustrated in FIG. 3, among four corners a, b, c, d atthe intersection of slot lines 4 a, 4 b, corners a, d positioned on theleft side of second slot 4 b are designated as one pair, while cornersb, c positioned on the right side of second slot line 4 b are likewisedesignated as another pair. A high frequency signal is applied betweencorners a, d and corners b, c. In this event, second slot line 4 b isexcited by the high frequency signal applied on both sides, permitting ahigh frequency component in a balanced mode to propagate through secondslot line 4 b. The high frequency component is converted to anunbalanced mode by microstrip lines 5 c, 5 d which traverse second slotline 4 b on the upper and lower end sides, respectively. The convertedhigh frequency component propagate to respective antenna elements 1.

[0054] Since the transition from the slot line to the microstrip line ismade through an anti-phase series branch, as is the case with theforegoing, the high frequency components converted by microstrip lines 5c, 5 d propagate in opposite phase, so that respective antenna elements1 are applied with the high frequency signal in opposite phase. However,since the feed points of the antennas are in a mirror symmetryrelationship, respective antenna elements 1 are excited in phase. Sincerespective antennas 1 are fed in the horizontal direction, a horizontalpolarization is supplied. Also, in this event, since first slot line 4 ais not excited, no high frequency component propagates throughmicrostrip lines 5 a, 5 b.

[0055] Further, as illustrated in FIG. 4, among four corners a, b, c, dat the intersection of slot lines 4 a, 4 b, a high frequency signal isapplied between corners d, b on one diagonal. Since corners d, b remainelectrically shut off to each other, an electric field is producedbetween corners b, c and corners c, d, while an electric field is alsoproduced between corners b, a and corners a, d, thereby excitingin-phase high frequency signals having the same amplitude on first andsecond slot lines 4 a, 4 b. As a result, the high frequency signalexcited on first slot line 4 a propagates through first slot line 4 a,and is branched in opposite phase and in series into microstrip lines 5a, 5 b which traverse first slot line 4 a on the left and right endsides, respectively. In this way, respective antenna elements 1 are fedin the vertical direction. Similarly, the high frequency signal excitedon second slot line 4 b propagates through second slot line 4 b, and isbranched in opposite phase and in series into microstrip lines 5 c, 5 dwhich traverse second slot line 4 b on the upper and lower end sides,respectively. As a result, respective antenna elements 1 are fed also inthe horizontal direction. Consequently, each antenna element 1 is fedwith the high frequency signals in both the vertical and horizontaldirections which are combined to form a linear polarization tilted by 45degrees to the right, as indicated by a long arrow.

[0056] When the high frequency signal is applied between corners a, c inthe diagonal direction opposite to the foregoing, instead of corners b,d, a linear polarization tilted by 45 degrees to the left, orthogonal tothe direction tilted by 45 degrees to the right, is formed on a similarprinciple to the foregoing.

[0057] More further, when the high frequency signal is supplied betweencorners b, d in one diagonal direction out of four corners a, b, c, d atthe intersection of slot lines 4 a, 4 b with an additional highfrequency signal being applied between corners a, c in the otherdiagonal direction, the planar antenna operates as follows. Each antennaelement 1 generates a linear polarization tilted by 45 degrees to theright in a similar manner to the foregoing by the high frequency signalapplied between corners b, d, and a linear polarization tilted by 45degrees to the left by the high frequency signal applied between cornersa, c. Here, if the high frequency signal applied between corners a, c isidentical in level and phase to the high frequency signal appliedbetween corners b, d, the linear polarization tilted by 45 degrees tothe right is combined with the linear polarization tilted by 45 degreesto the left to form a polarization substantially in the verticaldirection, i.e., a vertical polarization, as illustrated in FIG. 5.Thus, the linear polarization can be arbitrarily controlled in terms ofthe polarization direction by applying the high frequency signals atdifferent levels to each other.

[0058] While the multi-element planar array antenna according to thefirst embodiment has been described with particular emphasis on theoperation during transmission, the antenna operates in a manner similarto the foregoing during reception as well, as a matter of course. Also,while antenna element 1 is in the shape of a square, it can be in anyshape as long as the degeneration mode can exist in the orthogonaldirections. For example, the antenna element 1 can be formed of a slotline having a shape along a periphery of a rectangle or a circle onconductor 2 on the first principal surface of substrate 3.

[0059] Next, a slot line multi-element planar array antenna according toa second embodiment of the present invention will be described withreference to FIG. 6. This planar antenna is similar to the planarantenna according to the first embodiment except that the former isdesigned for use with a circular polarization.

[0060] The planar antenna illustrated in FIG. 6 largely differs from theplanar antenna according to the first embodiment in that the electriclength of first slot line 4 a from the intersection of slot lines 4 a, 4b to the point at which microstrip line 5 a, 5 b traverses first slotline 4 a is different from the electric length of second slot line 4 bfrom the intersection of slot lines 4 a, 4 b to the point at whichmicrostrip line 5 c, 5 d traverses second slot line 4 b by π/2 ascalculated in terms of phase difference. In this example, each antennaelement 1 is geometrically disposed at a corner of the square, and firstslot line 4 a extending in the horizontal direction is made longer thansecond slot line 4 b extending in the vertical direction. In FIG. 6, theelectric length from the intersection of slot lines 4 a, 4 b to theposition at which second slot line 4 b traverses microstrip lines 5 c, 5d is designated by α. Then, microstrip lines 5 a, 5 b extending in thevertical direction are bent in the shape of a crank to the outside in acentral portion thereof, while microstrip lines 5 c, 5 d extending inthe horizontal direction are bent in the shape of a crank to the insidein a central portion thereof. Each of microstrip lines 5 a to 5 d haveends electromagnetically coupled to associated antenna elements 1through substrate 3.

[0061] In the configuration as described above, a vertically excitedhigh frequency signal is delayed by π/2 in phase from a horizontallyexcited high frequency signal. Therefore, when the high frequency signalis applied between corners b, d, an electromagnetic wave propagating infront on the drawing sheet will be a right-handed circular polarization.Similarly, the high frequency signal applied between corners a, c willresult in a left-handed circular polarization. In addition, as the highfrequency signal is applied between corners b, d with additional highfrequency signal applied between corners a, c, a right-handed circularpolarization and a left handed circular polarization are excitedsimultaneously. In this manner, the right-handed circular polarizationor left-handed circular polarization can be selected depending on whichdiagonal direction is selected at the intersection of slot lines 4 a, 4b for applying the high frequency signal. Moreover, the right-handedcircular polarization and left-handed circular polarization can be usedtogether by applying the high frequency signal in both the diagonaldirections. Consequently, the second embodiment can readily implement aslot line multi-element planar array antenna which can select one fromorthogonal circular polarizations and can use these circularpolarizations together.

[0062] In the example shown herein, a difference of π/2 as calculated interms of a phase difference is provided between the electric lengths offirst and second slot lines 4 a, 4 b from the intersection, in whichcase the basic operation still remains unchanged when second slot line 4b extending in the vertical direction is made longer in the electriclength from the intersection by π/2 than first slot line 4 a extendingin the horizontal direction. Alternatively, slot lines 4 a, 4 b may beequal in the electric length, whereas a difference corresponding to aphase difference of π/2 may be provided between microstrip lines 5 a, 5b and microstrip line 5 c, 5 d. Moreover, it is still possible to selectone of the circular polarizations or use both the polarizations togetherwhen this difference in the electric length is appropriately distributedbetween the slot lines and microstrip lines as long as the differencebetween the electric lengths from the intersection of slot lines 4 a, 4b to two feed points of each antenna element 1 remains to be totally π/2as calculated in terms of phase difference.

[0063] Next, a slot line multi-element planar array antenna according toa third embodiment of the present invention will be described withreference to FIG. 7. The planar antennas in the respective embodimentsdescribed above are each configured to select a polarization componentand use together different polarization components at the same operatingfrequency of the antenna, whereas the planar antenna illustrated in FIG.7 is configured to use together different operating frequencies.Specifically, the planar antenna illustrated in FIG. 7 is similar to theone illustrated in FIGS. 2A and 2B except that a rectangular circuitconductor is used for antenna element 1. Antenna element 1 has a slotline which circumvents along the periphery of a rectangle having longsides and short sides. More specifically, antenna frequency f₁ in ahorizontal polarization resulting from the horizontal dimension ofantenna element 1 is different from antenna frequency f₂ in a verticalpolarization resulting from the vertical dimension. For convenience,suppose herein that antenna frequency f₂ is higher than antennafrequency f₁ (i.e., f₂>f₁) on the assumption that antenna element 1 islonger from side to side, as illustrated.

[0064] In the configuration as described above, as a high frequencysignal is applied between corners a, b and corners c, d, for example, atthe intersection of first and second slot lines 4 a, 4 b, first slotline 4 a extending in the horizontal direction is excited. Then, eachantenna element 1 is fed in the vertical direction through microstriplines 5 a, 5 b. Thus, the planar antenna can be operated at antennafrequency f₂ with the vertical polarization. Similarly, as a highfrequency signal is applied between corners a, d and corners b, c,second slot line 4 b extending in the vertical direction is excited, sothat each antenna element 1 is fed in the horizontal direction throughmicrostrip lines 5 c, 5 d. Thus, the planar antenna can be operated atantenna frequency f₁ with the horizontal polarization. From theforegoing, the resulting slot line multi-element planar array antennacan be operated at two frequencies selected through the orthogonallinear polarizations.

[0065] Preferably, in the planar antenna according to the thirdembodiment, both ends of first slot line 4 a extend beyond the positionsat which microstrip lines 5 a, 5 b traverse first slot line 4 a by onequarter wavelength with respect to antenna frequency f₂. Likewise, bothends of second slot line 4 b preferably extend beyond the positions atwhich microstrip lines 5 c, 5 d traverse second slot line 4 b by onequarter wavelength with respect to antenna frequency f₁.

[0066] Next, a slot line multi-element planar array antenna according toa fourth embodiment of the present invention will be described withreference to FIGS. 8A and 8B. Particularly shown herein is a specificmethod of feeding the slot line multi-element planar array antennahaving four antenna elements, illustrated in the first embodiment.

[0067]FIGS. 8A and 8B illustrate an example in which the planar antennais fed between corners b, d positioned in one diagonal direction at theintersection of first and second slot lines 4 a, 4 b on the firstprincipal surface of substrate 3. In this example, a feed microstripline 6 electromagnetically coupled to corners b, d is provided on thesecond principal surface of substrate 3 and for feeding a high frequencysignal to corners b, d through microstrip line 6. Microstrip line 6extends in the diagonal direction including corners b, d and passesimmediately above the position at which first and second slot lines 4 a,4 b intersect. The length from the intersection to an open end ofmicrostrip line 6 is set to approximately one quarter wavelength withrespect to a designed center frequency of the planar antenna. The otherend of microstrip line 6 is connected to feed connector 7 disposed onthe first principal surface of substrate 3 through a via hole. Forexample, a coaxial cable, not shown, is connected to feed connector 7.

[0068] In the configuration as described above, the planar array antennaaccording to the fourth embodiment can readily transmit theaforementioned linear polarization tilted by 45 degrees to the right byapplying a high frequency signal from the coaxial cable between cornersb, d in the one diagonal direction at the intersection of slot lines 4a, 4 b through feed microstrip line 6. Likewise, the planar arrayantenna can readily receive the linear polarization tilted by 45 degreesto the right in the same configuration. In addition, a similar feedmicrostrip line may be used for applying a high frequency signal betweencorners a, c, between corners a, b and corners c, d, and between cornersa, d and corners b, c. In these events, the planar array antenna can usea linear polarization tilted by 45 degrees to the left together with thelinear polarization tilted by 45 degrees to the right when feedmicrostrip lines are formed not only in one diagonal direction, i.e., inthe direction of corners b, d but also in the other diagonal direction,i.e., in the direction of corners a, c.

[0069] As described above, the multi-element planar array antenna basedon the present invention can be fed by simply disposing feed microstripline 6. This feature can be applied not only to the planar array antennaaccording to the first embodiment for use with a linear polarization butalso to the planar array antenna according to the second embodiment foruse with a circular polarization.

[0070] Next, a slot line four-element planar array antenna according toa fifth embodiment of the present invention will be described withreference to FIGS. 9A and 9B. In this planar antenna, functional circuit11, for example, a semiconductor device, an integrated circuit or thelike is mounted by surface mounting, flip chip bump technique or thelike, at the intersection of slot lines 4 a, 4 b on the first principalsurface of substrate 3 in the planar antenna of the first embodiment.Functional circuit 11 permits a high frequency signal to be selectivelyapplied to corners a, b, c, d at the intersection of the slot linesthrough functional circuit 11. In the illustrated configuration,functional circuit 11 is connected to the respective corners throughbumps 12.

[0071] In the configuration as described above, functional circuit 11may be controlled to facilitate a selection of applying a high frequencysignal between corners b, d; between corners a, c; between corners a, band corners c, d; and between corners a, d and corners b, c, therebyenabling the planar antenna to transmit and receive a polarizationtilted by 45 degrees to the right, a polarization tilted by 45 degree tothe left, a horizontal polarization, and a vertical polarization. Fromthe foregoing, the planar antenna according to the fifth embodiment canreadily select one from the linear polarizations listed above, and usesuch linear polarizations together. Generally, a millimeter-wavecommunication system suffers from a large loss on feed lines on top ofsmall power generated from an oscillation element. Such a problem on theloss can be solved by incorporating an active device such as anamplifier, a frequency converter and the like in the slot linemulti-element planar array antenna as functional circuit 11. Furtherfunctions can be added to the slot line multi-element planar arrayantenna to implement an active antenna or an adaptive array antenna. Inaddition, the configuration provided by the fifth embodiment is suitablefor a smart antenna for controlling a main beam and suppressinginterfering waves because of its ability to appropriately control andselect a polarization.

[0072] Next, a slot line multi-element planar array antenna according toa sixth embodiment of the present invention will be described withreference to FIGS. 10A and 10B.

[0073] This embodiment is similar to the fifth embodiment in that itshows a structure for feeding the multi-element planar array antenna.However, while the fifth embodiment has shown that substrate 3 having asingle layer structure is fed, the planar antenna according to the sixthembodiment comprises a feeder circuit in a multi-layered substrate whicheliminates via holes.

[0074] In the planar antenna according to the sixth embodiment, secondsubstrate 8 made of a dielectric material or the like is laminated onsubstrate 3 with one principal surface of second substrate 8 opposingthe first principal surface of substrate 3 in the planar antennaaccording to the first embodiment. Feed microstrip line 6 is formed onthe other principal surface of second substrate 8. Microstrip line 6 hasa leading end electromagnetically coupled to corners b, d in onediagonal direction at the intersection of slot lines 4 a, 4 b throughsecond substrate 8. The other end of microstrip line 6 is led to an endof second substrate 8 at which a coaxial cable, not shown, or the likeis connected.

[0075] In the configuration as described above, a high frequency signalis applied between corners b, d at the intersection of slot lines 4 a, 4b through feed microstrip line 6 provided on the principal surface ofthe multi-layered substrate, i.e., the other principal surface of secondsubstrate 8, enabling the planar antenna to transmit and receive theaforementioned linear polarization tilted by 45 degrees to the right. Inaddition, the elimination of via hole results in a suppressed reflectionloss and circuit loss. As will be appreciated, with additionalmicrostrip lines 6 thus provided, the resulting slot line multi-elementplanar array antenna of the sixth embodiment can transmit and receive alinear polarization tilted by 45 degrees to the left, a horizontalpolarization, and a vertical polarization as well as can use togethercircular polarizations and linear polarizations.

[0076] Further, in the sixth embodiment, the aforementioned functionalcircuit such as a semiconductor device and IC may be mounted on theother principal surface of second substrate 8, or a circuit boardcomprising a transmission line electromagnetically coupled to feedmicrostrip line 6 and a functional circuit may be laminated on secondsubstrate 8 to implement an active antenna or a smart antenna.

[0077] Next, a slot line multi-element planar array antenna according toa seventh embodiment of the present invention will be described withreference to FIGS. 11A and 11B.

[0078] In this planar antenna, second substrate 8 made of a dielectricmaterial or the like is laminated on substrate 3 with one principalsurface of second substrate 8 opposing the first principal surface ofsubstrate 3, on which slot lines 4 a, 4 b are formed, in the planarantenna illustrated in FIG. 7. Feed microstrip lines 13 a to 13 d areformed on the other principal surface of second substrate 8 such thatthey overlap first and second slot lines 4 a, 4 b within the region fromthe intersection of slot lines 4 a, 4 b to positions at which microstriplines 5 a to 5 d traverse these slot lines 4 a, 4 b.

[0079] In the configuration as described above, as a high frequencysignal is applied, for example, to feed microstrip lines 13 a, 13 cextending in the horizontal direction, an electric field is producedbetween corners a, d and corners b, c at the intersection of slot lines4 a, 4 b by electromagnetic coupling from microstrip lines 13 a, 13 c,thereby exciting second slot line 4 b extending in the verticaldirection as illustrated. Consequently, each antenna element 1 is fed inthe horizontal direction through microstrip lines 5 c, 5 d. Similarly,as a high frequency signal is applied to feed microstrip lines 13 b, 13d extending in the vertical direction, first slot line 4 a extending inthe horizontal direction is excited, so that each antenna element 1 isfed in the vertical direction through microstrip lines 5 a, 5 b.

[0080] While the seventh embodiment has illustrated a planar antennawhich has rectangular antenna elements 1 and operates at two antennafrequencies f₁, f₂, the seventh embodiment can be applied as well to aplanar antenna which has square antenna elements 1. Similar to theaforementioned sixth embodiment, the functional circuit such as asemiconductor device and IC may be mounted on the other principalsurface of second substrate 8, or a circuit board comprising atransmission line electromagnetically coupled to microstrip lines 13 ato 13 d and a functional circuit may be laminated on second substrate 8to readily implement an active antenna or a smart antenna.

[0081] Next, a slot line multi-element planar array antenna according toan eighth embodiment of the present invention will be described withreference to FIG. 12. The number of antenna elements in themulti-element planar array antenna of the present invention is notlimited to four, but any number of antenna elements such as 8, 16, 64and the like may be provided. Therefore, described herein is a16-element planar array antenna for use with a linear polarization basedon the present invention.

[0082] The planar antenna illustrated in FIG. 12 comprises four sets ofthe planar array antenna structures described in the first embodimentwhich are arranged on the same substrate 3 in 2×2 matrix configuration.Therefore, four sets of slot lines 4 a, 4 b as well as a total of 16slot line antenna elements 1 are formed on the first principal surfaceof substrate 3, while a total of 16 microstrip lines are formed on theprincipal surface of substrate 3.

[0083] A specific feeding method in the eighth embodiment may be, forexample, as follows. Feed slot line 9 extending in the horizontaldirection is disposed on a first principal surface of substrate 3between two upper sets and lower sets of four-element planar arrayantennas disposed as illustrated. Next, as described in connection withthe sixth embodiment, second substrate 8 is laminated on the firstprincipal surface of substrate 3, and feed microstrip lines 10 a to 10 care formed on the other principal surface, i.e., the exposed surface ofsecond substrate 8. Microstrip line 10 a traverses feed slot line 9 andis electromagnetically coupled thereto. Feed microstrip lines 10 b, 10 chave their central portions electromagnetically coupled to slot line 9on both end sides of slot line 9. Microstrip lines 10 b, 10 c have theirboth end sides electromagnetically coupled to the intersection of theslot lines in the upper and lower four-element planar array antennas,arranged side by side, for feeding between corners b, c, in a mannersimilar to microstrip line 6 (see FIGS. 10A and 10B) in the planarantenna of the sixth embodiment.

[0084] In the configuration as described above, a high frequency signalapplied from microstrip line 10 a is branched at the center of feed slotline 9 in parallel and in phase for distribution. Then, the highfrequency signal is branched in opposite phase and in series on both endsides of slot line 9, and distributed to microstrip lines 10 b, 10 c,respectively. Thus, the high frequency signal is applied in phasebetween corners b, c at the intersection of the slot lines in each ofthe four sets of four-element planar array antennas. In this manner, atotal of 16 antenna elements in the sets transmit and receive a linearpolarization tilted by 45 degrees to the right. The resulting 16-elementplanar array antenna provides a higher sensitivity.

[0085] While the foregoing description has been made on a 16-elementplanar array antenna, an 8-element planar array antenna can be providedby electromagnetically coupling one end of a feed slot line to themidpoint of microstrip line 10 b for feeding two sets of four-elementplanar array antennas disposed one above the other, and using the otherend of the feed slot line as a feed end. Also, a 32-element planar arrayantenna can be provided by disposing a pair of 16-element planar arrayantennas disposed one above the other in a mirror symmetry, commonlyconnecting microstrip lines 10 a of the respective 16-element allayantennas, and providing another feed slot line which traverses themidpoint of microstrip line 10 a and is electromagnetic coupled thereto.

[0086] While the 16-element planar array antenna described above isdesigned for use with a linear polarization, a 16-element planar arrayantenna for use with a circular polarization can be configured in asimilar manner by combining, for example, four sets of the planar arrayantennas of the second embodiment.

[0087] In the planar array antennas according to the respectiveembodiments of the present invention described above, electromagneticwaves are radiated from both principal surfaces of substrate 3. Forradiating an electromagnetic wave only from one of the principalsurfaces of substrate 3, an electromagnetic shielding box, a shieldingplate, a reflector or the like may be provided on the principal surfaceopposing to that from which the electromagnetic wave is irradiated.

What is claimed is:
 1. A multi-element planar array antenna comprising:a substrate having a first and a second principal surface; a conductorformed on said first principal surface; a first and a second slot lineformed in said conductor, and intersecting each other; a first and asecond microstrip line formed on said second principal surface, andtraversing said first slot line respectively at positions correspondingto both end sides of said first slot line; a third and a fourthmicrostrip line formed on said second principal surface, and traversingsaid second slot line respectively at positions corresponding to bothend sides of said second slot line; a first antenna elementelectromagnetically coupled to one end of said first microstrip line andto one end of said third microstrip line through said substrate; asecond antenna element electromagnetically coupled to one end of saidsecond microstrip line and to the other end of said third microstripline through said substrate; a third antenna element electromagneticallycoupled to the other end of said second microstrip line and to one endof said fourth microstrip line through said substrate; and a fourthantenna element electromagnetically coupled to the other end of saidfirst microstrip line and to the other end of said fourth microstripline through said substrate, wherein each of said antenna elements is aslot line antenna element formed on said first principal surface andcapable of being excited in two directions.
 2. The antenna according toclaim 1, wherein each of said antenna elements includes a slot lineformed on said conductor to be electromagnetically coupled to ends ofthe corresponding microstrip line.
 3. The antenna according to claim 2,wherein said slot line of each said antenna element is formed in a loopmanner.
 4. The antenna according to claim 1, wherein said antennaincludes a feed position at an intersection of said first and secondslot lines, through which a high frequency signal is applied between atleast two selected from four corners formed on said conductor at saidintersection to select an excitation mode for each said antenna element.5. The antenna according to claim 4, wherein said first and secondmicrostrip lines extend in directions orthogonal to each other, and saidfirst slot line matches with said second slot line at their respectivemidpoints to define said intersection.
 6. The antenna according to claim4, wherein each said antenna element has two feed points at which saidtwo microstrip lines are electromagnetically coupled to said antennaelement, respectively, and electric lengths are all equal from saidintersection to said respective feed points through said respective slotlines and said microstrip lines.
 7. The antenna according to claim 6,wherein said first and second slot lines are equal in length.
 8. Theantenna according to claim 6, wherein a high frequency signal is appliedon both sides of one of said first and second slot lines between a pairof said corners positioned on respective sides of said slot line at saidintersection.
 9. The antenna according to claim 6, wherein a highfrequency signal is applied between a pair of corners positioned in onediagonal direction out of said four corners at said intersection. 10.The antenna according to claim 6, wherein a first high frequency signalis applied between a pair of corners positioned in a first diagonaldirection, and a second high frequency signal is applied between a pairof corners positioned in a second diagonal direction different from saidfirst diagonal direction at said intersection.
 11. The antenna accordingto claim 6, wherein each said antenna element is in a shape of a square.12. The antenna according to claim 4, wherein each said antenna elementhas two feed points at which said two microstrip lines areelectromagnetically coupled to said antenna element, respectively, andan electric length from said intersection to one feed point through saidfirst slot line differs from an electric length from said intersectionto the other feed point through said second slot line by π/2 ascalculated in terms of phase.
 13. The antenna according to claim 12,wherein a high frequency signal is applied between a pair of cornerspositioned in one diagonal direction out of said four corners at saidintersection.
 14. The antenna according to claim 12, wherein a firsthigh frequency signal is applied between a pair of corners positioned ina first diagonal direction, and a second high frequency signal isapplied between a pair of corners positioned in a second diagonaldirection different from said first diagonal direction at saidintersection.
 15. The antenna according to claim 4, wherein each saidantenna element is in a shape of a rectangle, and a high frequencysignal is applied between a pair of corners positioned in one diagonaldirection out of said four corners at said intersection.
 16. The antennaaccording to claim 4, wherein each said antenna element is in a shape ofa rectangle, and a high frequency signal is selectively applied betweena first pair of corners positioned in a first diagonal direction orbetween a second pair of corners positioned in a second diagonaldirection different from said first diagonal direction out of said fourcorners at said intersection.
 17. The antenna according to claim 4,further comprising a feed microstrip line disposed on said secondprincipal surface and traversing said intersection.
 18. The antennaaccording to claim 4, further comprising a functional circuit disposedon said first principal surface and connected to said intersection forcontrolling a feed to said each corner.
 19. The antenna according toclaim 4, further comprising; a second substrate bonded on said secondprincipal surface, said second substrate having one principal surfaceopposing said second principal surface; and a feed microstrip linerouted on the other principal surface of said second substrate, andtraversing said intersection such that said feed microstrip line iselectromagnetically coupled to a pair of corners at said intersection.20. The antenna according to claim 4, further comprising: a secondsubstrate bonded on said second principal surface, said second substratehaving one principal surface opposing said second principal surface; anda first to a fourth feed microstrip line formed on the other principalsurface of said second substrate such that said microstrip lines overlapsaid first and second slot lines across said intersection.
 21. Amulti-element planar array antenna comprising: a substrate having afirst and a second principal surface; a conductor formed on said firstprincipal surface, two or more planar antenna units formed on saidsubstrate, said each planar antenna unit comprising a first and a secondslot line formed in said conductor, and intersecting each other; a firstand a second microstrip line formed on said second principal surface,and traversing said first slut line respectively at positionscorresponding to both end sides of said first slot line; a third and afourth microstrip line formed on said second principal surface; andtraversing said second slot line respectively at positions correspondingto both end sides of said second slot line; four slot line antennaelements formed respectively in intersection regions between both endsides of said first and second microstrip lines and both end sides ofsaid third and fourth microstrip lines, respectively, in two directionson said second principal surface; and a feed position at an intersectionof said first and second slot lines; a second substrate bonded on saidsecond principal surface, said second substrate having one principalsurface opposing said second principal surface; and a feed microstrip(line routed on the other principal surface of said second substrate,and traversing a pair of said intersections, wherein said antennaelements on said each planar antenna set are excited in phase.
 22. Theantenna according to claim 21, further comprising a feed slot lineformed on said conductor and electromagnetically coupled to said feedmicrostrip line.